Information recording media are known as a medium on and from which information can be recorded and reproduced at high capacity and high speed. Recording is performed utilizing the fact that a recording material is changed to have a different, optically-distinguishable state by the heat generated when the recording material is irradiated locally with a laser beam. Such information recording media can be accessed by random access, if needed, and also have excellent portability. With these great advantages, the information recording media further have grown in importance recently. For example, the demand is increasing in various situations, such as recording or storing of personal data and image information via computers, medical and academic fields, and replacing of home videotape recorders.
Today, phase-change optical media are needed to attain higher capacity, higher density and higher speed in accordance with improved performances of applications and image information.
Types of media that conventionally have been proposed include rewritable media capable of being written multiple times, and write-once media capable of being written only one time. Generally, since the write-once media can be provided with fewer layers compared with the rewritable media in many cases, they easily can be produced at low cost. Furthermore, since the write-once media are not rewritable, they are convenient for users when writing data that they do not want to destroy. For this reason, write-once media having a long storage life and a high reliability are in a great demand to be used for an archival purpose.
Some oxide materials are proposed as the write-once recording material. One of them is, for example, a recording material that includes TeOx (0<x<2), which is a mixture of Te and TeO2, as its main component (see JP50 (1975)-46317A). In the case of using the TeOx recording material, there was a problem that it took some time to obtain a state where the signals can be detected after being recorded. It has been reported, however, that the addition of an additive material such as Pd can deal with the problem (see JP58 (1983)-54338A). A recording mechanism of a Te—O—Pd recording material is conceived to be as follows. A Te—O—Pd film after having been formed is a composite material, wherein Te—Pd, Te, and/or Pd is uniformly dispersed in TeO2 as fine particles. After laser beam irradiation, the composite material is melted and Te, Te—Pd, and Pd are deposited as larger crystal particles, changing the optical state of the composite material. The difference between the optical states before and after the change can be detected as a signal. The recording mechanism possibly changes as the composition varies.
On the other hand, in recent years, a further improvement has been needed in the information recording media to have a higher recording density in response to the increasing volume of information. Therefore, it is necessary to develop a medium applicable for high density recording utilizing an optical system of a shorter wavelength and a higher numerical aperture (a higher NA), particularly, a blue-violet laser beam. In order to use the above-mentioned Te—O—Pd recording material in a blue-violet wavelength region, it has been proposed to increase the reflectance difference between a recorded region and an unrecorded region in an information layer by providing a dielectric layer adjacent to a recording layer (see JP2002-133712A).
In order to improve recording density further, multilayer information recording media have been proposed in which information can be recorded on a plurality of information layers. In such media, when recording data on an information layer disposed farther from the beam incident side, the laser beam needs to transverse information layer(s) disposed closer to the beam incident side. Therefore, the information layer(s) closer to the beam incident side has to ensure satisfactory signal quality while maintaining a high transmittance with respect to the laser beam. Since the material containing Te—O—Pd as its main component contains almost transparent TeO2 as a base material, it easily can increase the transmittance of the film. The material thus can be used as the recording material of multilayer optical information media in which information can be recorded on multiple of information layers by a laser beam incident from one side.
In an attempt of realizing a multilayer information recording medium including two or more information layers, recording and reproducing of information on and from an information layer located farther from the laser beam incident side may be affected by the information layer(s) located closer to the laser beam incident side.
That is, when the transmittance of the closer information layer(s) differs depending on the presence of a record, the amount of the laser beam to reach the information layer located farther varies depending on whether the closer information layer(s) is recorded or unrecorded. Therefore, it becomes difficult for the laser beam having been transmitted through the closer information layer(s) to record or reproduce accurately signals on or from the information layer located farther. This problem becomes more serious when a larger number of information layers are stacked. As an approach to solve such a problem, it has been proposed to minimize the transmittance difference by adjusting the material and thickness of the dielectric layers included in the information layers. However, there has been a problem that in write-once information recording media using a Te—O—Pd recording material, the transmittance difference cannot be made small enough only by adjusting the material and thickness of the dielectric layers included in the information layers.
In the write-once information recording medium described in JP2002-133712A using a Te—O—Pd recording material, in which a change from an amorphous state to a crystalline state is used as a recording principle, it is necessary to provide a dielectric layer on both sides of a recording layer, and to form a reflective layer on a side opposite to the laser beam incident side with respect to a recording layer so that the variation in reflectance shifts from high level to low level. Furthermore, this information recording medium successfully has realized satisfactory signal quality by having a structure in which the dielectric layers and the reflective layer are included in an information layer. At the same time, there has been a problem that one information layer needs to include at least four layers, leading to an increased production cost. This problem also becomes more serious when a larger number of information layers are stacked.
From the foregoing, in order to realize a multilayer information recording medium capable of accurate recording and reproducing of signals, an information recording medium strongly has been desired whose information layer closer to the beam incident side has a smaller transmittance difference and whose production cost is low.